This invention generally relates to circuit subassemblies, methods of manufacture of the circuit subassemblies, and articles formed therefrom, including circuits and multilayer circuits.
As used herein, a circuit subassembly is an article used in the manufacture of circuits and multilayer circuits, and includes circuit laminates, packaging substrate laminates, build-up materials, bond plies, resin coated conductive layers, and cover films. A circuit laminate is a type of circuit subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric substrate layer. Double clad laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating two or more materials together, at least one of which contains a circuit layer, using bond plies, in proper alignment using heat and/or pressure.
In use, a bond ply, or portions thereof, can flow and completely fill the space and provide adhesion between circuits, between a circuit and a conductive layer, between two conductive layers, or between a circuit and a dielectric layer. Thus, one or more of the polymers in a bond ply is designed to soften or flow during manufacture of the multilayer circuit but not in use of the circuit. In multilayer structures, after lamination, known hole-forming and plating technologies may be used to produce useful electrical pathways between conductive layers.
The optimum design of a bond ply for such applications would be a structure in which the composition of the bond ply is homogeneous throughout and has the same electrical, thermal, and mechanical properties (including low dielectric constant and low dissipation factor) as for the copper clad laminate. A bond ply used in the formation of rigid circuit laminates, multilayer circuits, and subassemblies, can also comprise a glass fabric saturated with an uncured or B-staged polymer composition, which cures in the circuit or subassembly lamination process. The glass fabric can provide a hard stop to prevent conductors on opposing layers from coming too close to each other causing low resistance or other problems.
Because bond plies, dielectric layers, and other circuit subassembly materials can contain synthetic organic materials having high carbon and hydrogen contents, they can be combustible. Many applications, however, demand that they meet flame retardancy requirements such as mandated in the building, electrical, transportation, mining and automotive industries. To meet these demands, circuit materials have included additives intended to interfere in various ways with the chemical exothermic chain of combustion.
In particular, compositions for circuit subassemblies have used halogenated, specifically brominated, flame retardant additives to achieve necessary levels of flame retardancy. In recent years, however, brominated flame retardants have come under increased scrutiny for their potential to contribute to health and environmental problems. There have been worldwide health and environmental concerns regarding brominated compounds because of their alleged potential to yield toxic by-products when burned or when disposed in landfills.
These concerns have spurred desires for ‘halogen-free’ circuit materials that have a UL94 flame retardance rating of V-1 or better, especially without bromine or chlorine. Some governmental or other groups have proposed that the specification for ‘halogen-free’ in a circuit material be less than 900 parts per million (ppm) of bromine, chlorine, or a combination thereof.
On the other hand, flame retardant additives that do not contain halogen can have serious drawbacks if used in circuit subassemblies, either because of their inherent properties or because they are less effective as flame retardants. The former drawback can lead to poor electrical properties, decreased thermal stability, and increased water absorption. The latter drawback might be overcome by use of very high loadings, but this can lead to deterioration of physical and/or electrical properties. Examples of alternative flame retardants that can cause such problems are some phosphorous compounds, aluminum trihydrate, borates, and the like. Also, some phosphorous or phosphinate based flame retardants are known to cause reduced copper peel strength, particularly with lower dielectric loss (Df)) core laminates.
It has, therefore, become highly desirable to develop a circuit subassembly that comprises an effective flame retardant, yet that contains essentially no halogens, especially bromine and chlorine. Consequently, a variety of organic phosphorous-containing compounds have been proposed or used as fire retardants in otherwise flammable resin compositions, because of their being perceived as more environmentally friendly. The compound's mechanism of action may be different than inorganic phosphorous-containing compounds, since the phosphorous content is significantly less, typically less than about 5-15 wt. %.
In particular, organo-phosphorous flame retardants with reactive groups (active hydrogens), such as those derived from 9,10-dihydro-9-oxa-10-phosphaphenantrene-10-oxide (“DOPO”), have been used in epoxy resin formulations and laminates. Such flame retardants are believed to react with the epoxy to form a phosphorus-modified epoxy resin. For example, US 2010/0234495 discloses the use of a mono-DOPO compound (having a single oxaphosphorinoxide group) and derivatives with other compounds. A salt of phosphinic acid is used as a synergist, and the exemplary compositions include a nitrogen-containing compound that is aminouracil. The amount of the DOPO compound in the examples is at most 7.5%. This reference discloses application to molded articles comprising various polymers, including molded articles comprising poybutadiene resins.
Several prior art references disclose the use of a DOPO compound (DOPO or a derivative thereof) in an epoxy-containing resin composition, apparently involving a reaction between epoxy groups in the resin and an active hydrogen in the DOPO compound. Prior art references showing such a combination include, for example, U.S. Pat. No. 6,291,627 (National Science Council in Taiwan), WO 2010/135393 (Abermarle), U.S. Pat. No. 6,524,709 (Matsushita), and US 2011/0054079 (Dow).
More recently, DOPO-derived flame retardants that do not have active hydrogen groups have been disclosed for use in various formulations, particularly as disclosed in WO 2011/123389 A1 and WO 2010/135398 A1. Working examples of DOPO-derived fire-retardants in combination with other than epoxy-containing resins formulations, however, have been lacking.
In the art of circuit laminates, epoxy-resin-based compositions for use in dielectric substrates are generally considered to be polar materials, which can be undesirable for circuit applications operating at high frequency or at high data speeds. High polarity can result in unacceptably high dielectric loss and/or other adverse effects.
Accordingly, there is a need for thermosetting compositions, especially otherwise highly flammable low polarity compositions, that contain an essentially non-halogen-containing fire retardant compound for providing effective flame retardant properties in a circuit subassembly without impairing physical properties or electrical properties, for example Df and moisture absorption properties. Finally, the fire retardant compound must be able to withstand the processing conditions of the circuit material, which can involve high temperatures and exposure to acid and/or alkali (low and/or high pH) solutions.